id like to suggest telling your parents that the fish will die from sickness if you dont change the water. though i dont know if that'll work given their interest in keeping up with water changes. they may say that they'll replcae the fish if it dies but its worth a shot to say that to them.
@dramaqueen: i had instances of my parents taking care of my pets when i was young ie 2 snapping turtles and 2 hamsters. me and my brother were too young to take care of them and hearing from the employee about seperate cages for the 2 hamsters. me and my brother wanted serperate cages but we got vetoed. i think some parents think they may know everything when its not the case or rather not go the extra mile to treat the animal better.
no offens to you Little Leaf, i wish you the best of luck in sneaking water changes for your fish.